well Obito also died when he was 13 so he would have had to had a huge growth spurt in a few short months and also had overcome his weakness to falling rock no jutsu
more likely its just madara's spirit in obitos body and if Phoenix Wright games have taught me anything being possessed by a spirit can change your physical appearance as well
but then you have to wonder how he could exist when Zombie Madara is currently fighting the Kages and he just revived himself completely recently didn't he?
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